1. Field of the Invention
The present invention relates to devices for indicating the presence of a noncondensable gas in steam sterilizers.
2. Description of the Prior Art
In prevacuum steam sterilizers, an evacuation stage during which air is removed from the chamber preceeds the sterilization stage during which the pressurized vapor, or steam, is injected into the chamber. Any residual air in the chamber tends to form an insulating shield around items placed in the sterilizer for processing. Such a shield of air may prevent the steam from effectively contacting all portions of the items. Thus, either the inefficient evacuation of the chamber or air leaks in the sterilizer may prevent acceptable sterilization.
In 1963, the Bowie-Dick test was developed for determining the air removal efficiency of prevacuum steam sterilizers. A cross of sterilizer tape is placed at various levels in a pack of muslin towels which, when folded into eight layers, forms a stack approximately 10-11 inches high. If a pocket of residual air forms, the tape registers the air's presence by a lack of change in the indicator. The air insulates the heat and humidity sensitive tape, thereby preventing at least a portion of the tape from undergoing the characteristic color change indicative of successful sterilization.
In place of sterilizer tape, sheets consisting of patterned heat and humidity sensitive indicators are placed midway in the pack of towels. A light spot towards the center of the sheet pattern indicates the presence of an air pocket. The Bowie-Dick pack provides three key parameters for the determination of sterilizer air removal: (1) a sufficient heat sink, (2) insulation properties, and (3) density impeding steam penetration. Upon contacting the pack, steam gives up its latent energy in heating the pack and collapses or condenses back to water. In the pack heat-up process, steam collapses repeatedly until the pack temperature reaches that of steam. In the collapse of steam, a void is created which is instantaneously filled by the surrounding steam. If air is mixed with the steam, the air will remain after each collapse of the steam, thus creating a concentration of air. The Bowie-Dick towel pack also absorbs the condensed steam, adding to its heat sink capacity, and not interfering with the chemical indicator. The towels of the pack themselves are poor heat conductors and are primarily heated through actual steam penetration. The towel pack density limits the heat-up process to a moving front where the pack is heated progressively towards the center rather than uniformly throughout the pack. If an air leak is present or an insufficient vacuum is drawn, during the evacuation stage, air is forced into and/or remains in the pack. In the final stage of the cycle, steam is injected into the sterilizer and the temperature is maintained at about 270.degree. F. At this point, if residual air is still present, it is concentrated toward the center of the pack where it prevents the steam from contacting the indicator. The area of chemical indicator sheet insulated by the air does not change color while the exposed sections turn black.
There is no definite standard for the Bowie-Dick test. Interpretation of the tape or test sheet color gradations is subjective. Furthermore, inconsistent thread count in the muslin towels, variations in pack penetration and laundering can alter the resistance to steam penetration from test to test so the color changes are inconsistent for the same quantities of air. The inconsistent results over time, together with the subjective nature of the interpretation, create a margin of error at critical lower levels of an air leak.
Joslyn U.S. Pat. No. 4,115,068, which issued on Sept. 19, 1978, discloses an air detecting device for steam or gas sterilizers. The Joslyn air detector includes an upright insulated tube, closed at the top and open at the bottom, a heat sink lining the inside of the tube for condensing the steam, and a thermal indicator strip in the tube spaced from the heat sink. Steam enters the tube at the bottom, travels an upward path to the chamber where it is condensed in close proximity to the indicator. The air is entrapped at the top of the tube and the condensate falls down the tube to exit the device. Because the condensate forms in the same chamber in which the indicator strip is located, the condensate can contact the strip. The heat from incoming steam can vaporize the condensate on the strip, thereby interfering with the shielding effect of the air. The penetration gradient in the Joslyn air detector is from bottom to top. Incoming steam must work against gravity, the weight of any accumulated air, which is heavier than steam, and the downward moving condensate.
There is a need for a device for indicating the presence of noncondensable gas, or air, in a steam sterilization chamber which will provide standardized results to reduce the margin of error associated with the prior art tests. There is a further need for such a device which reduces the opportunity for interference with the insulating effect of the noncondensable gas. Finally, there is a need for a device which is sensitive enough to detect critical quantities of air.